1. Field of the Invention
The present invention relates to an arrangement for supporting a shaft of a vacuum pump having a housing, and including a first bearing for supporting the shaft at one of its ends and generating forces in a direction of a shaft axis and having an axial stiffness and a second bearing for supporting the shaft at another of its ends and formed as a rolling bearing. The arrangement further includes a holder for receiving the second bearing and having an axial stiffness and a radial stiffness.
2. Description of the Prior Art
In turbomolecular pumps, shortly, turbopumps, a rapidly rotating rotor produces, in cooperation with a stator, a pumping effect. The rotor should be adequately supported, and different methods have been developed to this end. A successful commercial method includes using a rolling bearing at a forevacuum side and a permanent magnetic bearing at a high vacuum side end of the rotor shaft.
The permanent magnetic bearing is formed of ring-shaped, axially magnetized permanent magnets, with the magnetic rings being axially stacked upon each other. In the stack, magnetic rings, which have opposite polarities, are lying over each other. Two stacks with the same stack sequence of polarities but having different diameters are used, with respective stacks being mounted on the rotor and the stator.
In the normal position, the magnetic rings of both stacks are arranged at the same height, i.e., concentrically and coplannar. Because each layer of the stack has the same polarity, the support is based on the repelling effect. This has, however, an undesirable side effect. If the rotor is loaded axially and becomes deflected from its normal position, the magnetic rings are also displaced from their normal position. The repelling action in this case, produces additional axial forces. These additional forces increase the load acting on the forevacuum side rolling bearing and its axial support.
The load, which is applied to the forevacuum side rolling bearing, is critical for the following reasons: the bearing should be supported in the housing of the turbopump with a possibility of oscillating movement in order to keep the transmission of vibration low. Therefore, the oscillating ability of the bearing, on one hand, and the resistance against axial forces, on the other hand, present contradictory requirements.
Each elastically deformable structure can be associated with stiffness. This applies to a permanent magnetic bearing as well as to the mechanical components, e.g., to vibrating rings. In case of a permanent magnetic bearing, the generation of the axial force upon deflection from the normal position can be characterized by its stiffness.
The rolling bearing or its holder up to the present was embedded in an elastomeric material, so that the oscillating ability was provided in both axial and radial directions. However, the contradictory requirements cannot be taken into account in this way or can be taken into account only to an insufficient extent. In addition, the oscillating ability depends to a large extent on the elastomeric material itself.
Accordingly, an object of the invention is to provide an arrangement for supporting a shaft capable to meet these contradictory requirements.